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scoutfs: fix item cache page split key choice

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The algorithm for choosing the split key assumed that there were
multiple items in the page.  That wasn't always true and it could result
in choosing the first item as the split key, which could end up
decrementing the left page's end key before it's start key.

We've since added compaction to the paths that split pages so we now
guarantee that we have at least two items in the page being split.  With
that we can be sure to use the second item's key and ensure that we're
never creating invalid keys for the pages created by the split.

Signed-off-by: Zach Brown <zab@versity.com>
This commit is contained in:
Zach Brown
2020-10-08 12:15:11 -07:00
committed by Zach Brown
parent 27bc0ef095
commit 8bf4c078df
1 changed files with 15 additions and 4 deletions
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19
kmod/src/item.c
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@@ -1053,9 +1053,14 @@ static void drop_pcpu_pages(struct super_block *sb,
}
/*
* We're about to move all the items between to a pair of new pages.
* Find the item that balances the space consumed by items in either
* page. We move the mid (and possibly only) item to the right page.
* Set the keys of the destination pages of a split. We try to find the
* key which balances the space consumed by items in the resulting split
* pages. We move the split key to the right, setting the left end by
* decrementing that key. We bias towards advancing the left item first
* so that we don't use it and possibly decrementing the starting page
* key. We can't have a page that covers a single key. Callers of
* split should have tried compacting which ensures that if we split we
* must have multiple items, even if they all have the max value length.
*/
static void set_split_keys(struct cached_page *pg, struct cached_page *left,
struct cached_page *right)
@@ -1066,8 +1071,14 @@ static void set_split_keys(struct cached_page *pg, struct cached_page *left,
int left_tot = 0;
int right_tot = 0;
BUILD_BUG_ON((PAGE_SIZE / SCOUTFS_MAX_VAL_SIZE) < 4);
BUG_ON(scoutfs_key_compare(&pg->start, &pg->end) > 0);
BUG_ON(left_item == NULL);
BUG_ON(right_item == NULL);
BUG_ON(left_item == right_item);
while (left_item && right_item && left_item != right_item) {
if (left_tot < right_tot) {
if (left_tot <= right_tot) {
left_tot += item_val_bytes(left_item->val_len);
left_item = next_item(left_item);
} else {